The order Thursday by the Federal Aviation Administration gave operators seven days or five flight hours to inspect two sets of fuel pump electrical lines in each wing.
Based on early inspection results reported Saturday, the FAA made the directive immediate but said only one type of power line needed to be checked.
Operators have completed inspections of only 13 of the affected aircraft, FAA spokesman Eliot Brenner said. Those inspections found signs of wear on the insulation of wires in 13 of 26 main pump linesÂ—the one now subject to immediate checking and repair.
One wiring bundle on a United Airlines 737 showed "clear signs" of arcing in one location and a second spot where bare wire already was exposed, the FAA said in a statement.
The early inspections did not find any damage to wiring for the center fuel pump, so the FAA decide to exclude those electric lines from further checking in planes with at least 50,000 flight hours.
Based on the original order, the FAA estimated an inspection would take two mechanics 15 hours to complete.
Brenner said in an interview he was hearing airlines "can do it more quickly" than the agency first estimated. "Narrowing the scope should make it quicker," he said.
Asked how the revised inspection might affect flight schedules for carriers with the 737-100 and -200 series, he said it was a matter of "how quickly the airlines can perform the work."
Also Sunday, the FAA ordered inspections for both sets of electrical lines in 737s with between 40,000 and 50,000 flight hours. That affects later models of the 300, 400 and 500 series of the Boeing plane. Operators have 14 days to complete the work.
The FAA said the expanded order covers an additional 118 domestic-registered planes and 282 worldwide, including U.S. aircraft. The agency said there are 1,088 Boeing 737s registered in the United States and 2,716 worldwide.
The FAA had acted Thursday after mechanics working on a Continental Airlines Boeing 737-200 found fuel leaking out of a pipe running through one of the plane's wing fuel tanks.
Upon closer examination, the mechanics found that the fuel got into the tubing through holes that were apparently made by electrical arcing. The aircraft was subsequently grounded, repaired and returned to service.